Internal antennas have been used for some time in portable radio communication devices. There are a number of advantages connected with using internal antennas, of which can be mentioned that they are small and light, making them suitable for applications wherein size and weight are of importance, such as in mobile phones. A type of internal antenna that is often used in portable radio communication devices is the monopole antenna.
However, the monopole antenna is inherently resonant in one frequency band. If multi-band operation is required, wherein the antenna is adapted to operate in two or more spaced apart frequency bands, two monopole antennas with different resonance frequencies can be provided. In a typical dual band phone, the lower frequency band is centered on 900 MHz, the so-called GSM 900 band, whereas the upper frequency band is centered around 1800 or 1900 MHz, the DCS and PCS band, respectively. If the upper frequency band of the antenna device is made wide enough, covering both the 1800 and 1900 MHz bands, a phone operating in three different standard bands is obtained. However, with today's high demands on functionality, antenna devices operating four or even more different frequency bands are in demand. With the limitations regarding cost and size of antenna devices this quad band operation is difficult to achieve.
A problem in prior art antenna devices is thus to provide a multi-band antenna with a small size and volume and broad frequency bands which retains good performance.